The present invention relates to the field of marine pipeline installation.
From the prior art reel lay vessels are known for laying a pipeline on the seabed according to the reel lay method.
These vessels commonly are equipped with one or more storage reels for storage of pipeline to be installed. The vessels also are equipped with a pipeline launch tower adapted for launching the pipeline in the direction of the seabed. Such a tower is often placed adjacent or over a moonpool of the vessel, or at another suitable location, e.g. at the stern of the vessel. In general the term tower here refers to a raised structure on the vessel.
It is also known to provide a pipeline guide at an elevated position on said tower providing a curved path for guiding said pipeline toward a pipeline launch trajectory along or within said tower. Known pipeline guides include a large diameter pipeline guide wheel or a semi-circular guide structure, e.g. provided with rollers or chains.
In known reel lay pipelaying systems one or more pipeline tensioners, commonly two but versions with one or three are also known, are supported by the tower at different heights below the curved or circular pipeline guide.
In a common design each pipeline tensioner has a tensioner frame and multiple tracks (also called caterpillars), often three or four tracks, supported by said tensioner frame. The tracks are adapted for engaging the pipeline and supporting at least part of the weight of the launched pipeline. In practice a single tensioner can be designed to support a pipeline weight of more than 50 tons, and tensioner capacities of more than 100 tons are also not uncommon.
The tracks are mounted in said tensioner frame in a movable manner in order to allow for adjustment of the gap between the tracks to accommodate various pipeline diameters.
It is also known from the prior art to design the tensioner frame “oversized” with respect to regular pipeline diameters, so that the gap between the tracks can be increased to a large size which allows for the passage of an accessory fitted on (the end of) or in the pipeline, e.g. a connector on the end of or in a flexible pipeline. Such connectors are bulky and have a diameter substantially greater than the pipeline diameter, e.g. having a diameter of more than 1 metre for connectors in flexible offshore pipeline.
The large loads acting on the tensioners require a sturdy design of the tensioner frame and also of the associated support assembly via which the tensioner frame is connected to the tower.